This invention relates to a process of forming composite structural members and more particularly to such a process using plies of fibers and resin in the uncured state and wherein preformed sheets of thermoplastic material are used as a tool and as a vacuum bag and which become an integral part of the structural member upon curing of the resin.
In U.S. Pat. No. 3,959,544 filed by me on Dec. 26, 1973 and entitled "Filamentary Tape Construction and Methods" there is disclosed structural members fabricated from a tape formed from plies of unidirectional fibers impregnated with a matrix such as a thermosetting resin in the uncured state. In the embodiment disclosed, the structural members formed are employed to form a warehouse cart. The fibers preferably are of fiber glass although they may be formed of graphite or boron. In an earlier process of fabricating the structural members, the tape is laid in a desired manner against a rigid preformed tool surface to form one side of the structure. Foam stiffner forms are laid over the tape at spaced apart positions and an additional layer of tape is laid over the foam stiffner forms and over the first layer of tape. A thin film vacuum bag next is placed over the lay-up and sealed to the tool and a vacuum drawn. The vacuum bag is formed of a material that does not stick to the resin used. The resin then is allowed to cure and after curing the vacuum bag is then removed and discarded.
Although carts formed from such structural members are relatively light weight and have the desired strength, thickness, and serviceability, the manufacturing process has some disadvantages. For example the finished carts are crude looking since the foam stiffner forms are sometimes irregularly spaced. This is due to the fact that there is no positive way to locate the foam stiffner forms during the lay-up operation. In addition, the time required to place the tape on the "floating stiffner forms" is longer than desired. Since the thin film vacuum bag is used only once and then discarded, the cost is higher than desired. Moreover, since the thin film vacuum bag must fit loosely, it wrinkles when the vacuum is achieved. These wrinkles cause unsightly resin ridges. In order to minimize these wrinkles, considerable time and effort must be spent by the workman hand rubbing and otherwise moving the wrinkles to regions of the part where they are not as noticeable. On portions of the structural members which are curved, small pits or air pockets are formed. This is due to the fact that the tape is relatively coarse, trapping large amounts of air all of which is not removed during the vacuum cure process.